This invention relates to a lead bearing an electrode for connecting a living organ to an electrical device. Notwithstanding its various uses, this invention will be described as an endocardial pacing and sensing lead for connecting a pacemaker to cardiac tissue.
There are generally two types of body-implantable leads--one which requires surgery to expose that portion of the body to which the electrode is to be affixed and the other which is inserted in and guided to the desired location through a body vessel such as a vein. In the cardiovascular field, in particular, there are myocardial and endocardial type leads. Use of a standard myocardial lead such as that disclosed in U.S. Pat. No. 3,216,424, generally provides an excellent electrical contact but requires a thoracotomy in order to affix the electrodes in the outer wall of the heart. This type of surgery is quite strenuous on the patient, particularly an elderly one. Even the improved myocardial leads, e.g., the type disclosed in U.S. Pat. Nos. 3,416,534, 3,472,234 and 3,737,579, require a minor transthoracic surgery to obtain access to the myocardium in order to screw the electrode in place in heart tissue with a special tool or surgical instrument.
Use of a standard endocardial lead of the type shown in U.S. Pat. No. 3,348,548, for example, does not involve serious surgery since the lead is inserted in and guided through a selected vein. However, endocardial leads currently in use are difficult to place and to maintain in proper position and do not insure the best electrical contact since the electrode merely rests against the inner wall of the heart or endocardium at or near the apex of the right ventricle. As a result, the electrodes of such prior art leads tend to become dislodged from their proper position, often resulting in loss of heart capture and thus loss of stimulation of the patient's heart. Also, since the electrodes of an endocardial lead are not secured in the cardiac tissue, the lead tends to move with each contraction of the heart muscle, thereby forming an undesirable callous or fibrotic growth on the inner wall of the right ventricle. Another problem is that with the contraction of the heart, the tip or distal electrode may occasionally puncture the heart wall, resulting in serious injury to the heart and a loss of heart capture.
Many attempts have been made to develop an endocardial lead that can be simply and reliably secured for chronic pacing through endocardial tissue. Typical of such lead designs are those disclosed in U.S. Pat. Nos. 3,754,555 and 3,814,104 which involve a mechanism carried within the lead for advancing prongs or hooks from recesses in the distal end of the lead into endocardial tissue after the lead has been transvenously advanced and positioned within the heart. A further variation on this approach involves the use of a hollow sleeve or introducer catheter of the same length as the lead to shroud the electrode while it is advanced transvenously into the desired position in the heart, whereupon the electrode is advanced from the sleeve or catheter introducer into endocardial tissue. Typical of these latter designs are those disclosed in U.S. Pat. No. 3,844,292 and in the article entitled "New Pacemaker Electrodes" by Max Schaldock appearing in Vol. 17 Transactions: American Society for Artificial Internal Organs, 1971, pp. 29-35.
These prior art endocardial lead designs have not been completely successful in achieving the objects of reliable chronic securement in the endocardial tissue. The prongs or hooks of the former type at times fail to remain in place, or become caught in trabecular cardiac tissue and the electrode remains displaced from and in poor electrical contact with the endocardium. The catheters or sleeves of the latter type add undesirable bulk to the lead as it is advanced through the vein and its increased stiffness makes positioning the electrode tip in the desired location in the heart difficult. In both instances, the complexity of such leads reduces their statistical reliability while raising their cost. If the implanting surgeon should erroneously advance the prongs or hooks from their recesses in the electrode tips or if the same occurs through a malfunction of the lead during advancement of the lead through the veins and heart valves, serious injury could occur as the prongs or hooks snag the valves or the tissue lining the veins.
The body-implantable lead of the present invention combines all the advantages of both the myocardial and endocardial leads with none of the attendant disadvantages of each of these leads as currently found in the prior art. One of the features of the present invention is the provision of a body-implantable intravascular lead which can be lodged in and permanently secured to or removed from the body tissue which it is desired to stimulate, without the use of complex electrode advancement mechanisms or bulky sleeves or catheter introducers. Another feature of the invention is that the body of the lead exclusive of the novel electrode comprises a reliable design that enjoys demonstrated reliability in chronic use and is easily placed in the heart according to well known and proven techniques.